The present invention generally relates to dipole antenna structures and more particulary to a dual band dipole antenna structure operative to efficiently transmit radio frequency (RF) energy at two different frequencies.
In order to efficiently operate, the length of a dipole antenna is typically related to the operating frequency thereof. The length of the dipole element is a multiple of the frequency to be transmitted or received. For example, the dipole element may have a length that is xc2xc, xc2xd, or xc2xe the wavelength of transmission. As will be recognized, a single dipole element cannot efficiently operate for multiple operating frequencies because the length thereof must change.
For instance, in wireless technology, the device may need to operate on two different frequency bands. The device may have an operating frequency of either 800 MHZ or 1900 MHZ depending upon the type of service the wireless device is accessing. As such, the antenna structure must be capable of efficient transmission and reception of RF energy at both of those bands.
Printed antenna structures are widely used to provide compact antennas for portable devices. The printed antenna structures are typically formed on a substrate such as a PCB by forming conductive traces on the PCB. In this regard, the printed antenna structure can be integrated with other electronic devices on the substrate. Typically, the antenna structure is designed on a rigid PCB having a thickness of about 3-5 mm. Therefore, the size and thickness of the PCB restrict the size of the device that the antenna can be placed within. Typically, in portable wireless devices (i.e., cellular telephones), the housing for the device is designed around the size of the antenna structure.
In order to efficiently transmit over both frequency bands, printed antenna structures have been designed with complicated wire patterns in order to provide the correct dipole length. For instance, in U.S. Pat. No. 5,949,383 to Hayes et al. entitled xe2x80x9cCompact Antenna Structures Including Balunsxe2x80x9d, the printed antenna structure includes multiple radiating sections and a balun in order to tune the antenna for two operating frequencies. The printed antenna structure further includes a tunning shunt across the balun in order to provide dual band operation. In this sense, the printed antenna structure includes a complicated trace structure and tunning mechanism to provide dual band operation.
The present invention addresses the above-mentioned deficiencies in the prior art antenna structures by providing a dipole antenna structure that is compact in size and easily formed. More specifically, the present invention provides an antenna structure that is formed on a thin film PCB and comprises two dipole elements and corresponding dipole grounds. In this sense, the design of the antenna structure for the present invention provides for dual band operation with a compact and easily fabricated structure.
In accordance with the present invention, there is provided a dual band antenna structure having a substrate with first and second sides. The first side includes a first dipole element, and a second dipole element formed in substantially parallel relation to the first dipole element and electrically connected thereto. The first side of the antenna further includes a generally wedged shaped transformer electrically connected to the first and second dipole elements. The second side of the antenna structure includes a first dipole ground disposed in generally opposite relation to the first dipole element and a second dipole ground disposed in generally opposite relation to the second dipole element. The first and second dipole grounds are electrically connected together via a ground line. Accordingly, RF energy fed into the transformer can be transmitted at a first frequency by the first dipole element and can be transmitted at a second frequency by the second dipole element.
In accordance with the present invention, the first dipole element has a length equal to about xc2xc the wavelength of the first frequency and the second dipole element has a length equal to about xc2xc the length of the second frequency. The first dipole ground has a length equal to about xc2xc the wavelength of the first frequency, while the second dipole ground has a length equal to about xc2xc the length of the second frequency. Both the first and second dipole elements are disposed in substantially parallel relation to the transformer element.
In the preferred embodiment, the shape of the first dipole ground is substantially similar to the shape of the first dipole element, while the shape of the second dipole ground is substantially similar to the shape of the second dipole element. In this respect, both the first dipole element and the second dipole radiating element are substantially rectangular. The first and second dipole grounds are disposed in opposite relation on the second side of the substrate in substantially mirror-image relation to respective first and second dipole elements.
In accordance with the present invention, the substrate is a thin film such as a thin film PCB. The thin film may additionally be flexible. The first and second dipole elements are formed as conductive tracings on the PCB through conventional techniques. A microstrip is formed as the ground line connecting the first and second dipole grounds, as well as to connect the first dipole element, the second dipole element and the transformer.
In accordance with the present invention, there is provided a dual band antenna structure having a substrate, a first antenna array, a second antenna array, and a transformer. The first antenna array has a first dipole element disposed on a first side of the substrate. Furthermore, the first antenna array has a first dipole ground disposed on a second side of the substrate. The first dipole ground is disposed in substantially mirror-image relationship to the first dipole element. The second antenna array has a second dipole element disposed on the first side of the substrate and a second dipole ground disposed on the second side of the substrate. The second dipole ground is disposed in substantially mirror-image relationship to the first dipole element. The transformer is formed on the first side of the substrate and electrically connects the first and second dipole elements. In this respect, the first array is operative to transmit electromagnetic energy at a first frequency and the second array is operative to transmit electromagnetic energy at a second frequency when the electromagnetic energy is fed to the transformer. The length of the first dipole element is chosen to transmit the first frequency and the length of the second dipole element is chosen to transmit the second frequency.
In accordance with the present invention, there is provided a method of forming a dual band antenna structure for transmitting a first and a second frequency. The method comprises providing a thin film substrate having a first side and a second side. Next a first dipole element is formed on the first side of the substrate. A first dipole ground is formed on the second side of the substrate in substantially mirror-image relation to the first dipole element. A second dipole element is formed on the first side of the substrate and a second dipole ground is formed on the second side of the substrate in substantially mirror-image relation to the second dipole element. Finally a transformer is formed on the first side of the substrate. The transformer is electrically connected to the first dipole element and the second dipole radiating element.